There are two groups that make recommendations and set standards for the lighting of roadways in the United States. These are the Illumination Engineering Society of North America (IESNA) in conjunction with the American National Standard Institute (ANSI), which publishes the American National Standard Practice for Roadway Lighting designated ANSI/IESNA RP-8-2000, and the American Association of State Highway and Transportation Officials (AASHTO), which publishes the “Roadway Lighting Design Guide”. Moreover, the Federal Highway Administration (FHWA) has a manual for roadways, namely The Roadway Lighting Handbook 1978 (revised 1984), that includes lighting as do the fifty states which use it as the basis for their respective state practices. Some states use RP-8 and some use AASHTO, and some states have no written uniform policies, instead leaving lighting practice up to district offices. The FHWA has a policy of funding projects that use the AASHTO guide or ANSI/IESNA RP-8. The criteria for lighting freeways do not differ significantly between RP-8 and AASHTO for the Illuminance or Luminance Methods.
One of the major criteria recommended by both groups is the “full cutoff” requirement. This establishes limits on light emitted vertically from the luminaire to minimize “skyglow.” Skyglow is the result of light directed upward into the atmosphere and is an undesired effect of outdoor lighting. It consists both of light from the luminaire that is reflected by the ground or other structures, as well as the direct light from the luminaire directed outward at near-horizontal angles and upward.
“Full cutoff,” as defined by IESNA, requires that there be no direct light from the luminaire at or above the 90° vertical angle and no more than 10% of the lamp lumens at or above 80° vertical angle. This is interpreted by reviewing the candlepower distribution table of a photometric report of the luminaire and not finding any candlepower value in the angle ranges greater than 10% of the rated lamp lumens. For example, if a 400 watt HPS lamp is rated 50,000 lumens, then a candlepower value of 5000 or more at or above 80° vertical angle in any lateral plane would exceed the limit for this classification.
Conventional roadway lighting systems include roadway luminaires that provide a bisymmetric lighting distribution. This means that an equal amount of light is projected from the luminaire upstream and downstream of the traffic flow on the roadway. In other words, there is a light distribution symmetry about a plane perpendicular to the axis of the roadway. However, this light distribution results in glare and alternating areas of positive (in the direction of traffic) and negative (against the direction of traffic) light contrast.
Another standard practice is to space lighting poles at four to six (4-6), and more typically five (5), “mounting heights.” As the term implies, “mounting height” is the height above ground at which the luminaire of the light pole is mounted. Thus, if the common mounting height is 40 feet above ground, the spacing between poles will typically be 160-240 feet. Typical mounting arrangements for roadway poles include single roadside arrangements, staggered double-sided arrangements, opposite double-sided arrangements and median mounted arrangements.
It is also important to light the right hand edge of the road to help the driver keep his vehicle on the road and to help identify exit and entrance ramps, as well as merging traffic. Slower traffic having mechanical problems typically travel to the right and animal and pedestrian intrusion also occurs in this area. For these reasons it is desirable for the luminaire to target light on the two most right hand lanes and the breakdown lane and let spill light take care of the inner lanes.
Other standard criteria for lighting roadways include minimum maintained average values for illuminance, luminance and Small Target Visibility (STV in RP-8). Each of these has a uniformity requirement as well as a requirement for comparing the average to the minimum and the maximum to the minimum. There also exists a glare measure called the veiling luminance ratio. This compares the cumulative contribution of the luminance of all of the luminaires to the background scene or pavement luminance.
Accordingly, it would be desirable to provide a roadway lighting system that meets as many of the illuminance, luminance, STV, uniformity, glare, full cutoff, spacing and utilization criteria as possible. It is also desirable to provide a system utilizing a “Positive Contrast” light distribution to improve visibility, complement vehicle headlights and minimize glare to the driver during the driving task, wherein the targeted veiling luminance ratio is 0.20:1.0 or less.
It would further be preferable to provide a luminaire having a size and shape that conforms to products that are currently on the market so as to blend in when used as a replacement in existing installations. The overall weight and EPA of the luminaire should be similar as well so as to not exceed ratings for existing poles and arms and to permit the retrofitting of luminaires with better performing, energy saving replacements.